Teeth contain a record of some of the physiological changes that occur during dental development. Because of this record they provide an important resource for studying aspects of health and growth. An important histological marker of dental development is the striae of Retzius (SR), a series of regularlyoccurring lines in enamel. However, the biology of this dental feature is poorly understood. SR are formed when unknown factors repeatedly cause changes in the way that enamel matrix is laid down. They occur with a constant periodicity in an individual's teeth, but periodicity varies across individuals with a range of 6 to 12 days. Predictable periodicity makes SR useful for estimating the timing of dental development, but more information is needed on the factors that influence SR before the full potential of this feature can be realized. The overarching goal of this exploratory investigation is to identify some of the factors that affect SR periodicity. We have two specific aims. The first is to test the heritability of SR periodicity using shed deciduous teeth from identical and fraternal twin pairs. This test will indicate the extent to which variation in periodicity is the result of genetic variation. If genetic variation is a major factor, additional research would be justified to determine the genes underlying SR formation. Our second aim is to test for associations between periodicity and basic biological and environmental characteristics including sex, birth weight, current height and weight, metabolism, health history and climate. Linkages that emerge will provide clues about the relative importance of genetic and environmental factors in the formation of SR. We will recruit 125 identical and 125 same-sex fraternal twin pairs through the National Organization of Mothers of Twins Clubs. For each twin, we will collect: 1) shed deciduous teeth, to determine periodicity; 2) DMA samples (as needed) to determine twin zygosity; and 3) questionnaire data, to identify associations between periodicity and biological, environmental or health factors. We will estimate heritability using maximum likelihood models and test for associations between periodicity and other characteristics using ANOVA/ANCOVA methods. This research on SR biology will be relevant to studies of human evolution and health; enamel development, microstructure, and functionality; and perhaps even enamel strength and caries risk.